System for operating a fluid actuator

ABSTRACT

A system is disclosed for driving an actuator which comprises a chamber, a shutter of an injection nozzle for molten material for injection molding, and a piston translatable inside the chamber to actuate a shutter. Means are provided for the transfer of a predetermined amount of fluid between the outside and inside of the chamber, so as to bring/move the shutter from a closing position, in which there is no passage of molten material, towards an opening position, in which there is passage of molten material, by injecting the predetermined amount of fluid from the outside into the chamber or by injecting the predetermined amount of fluid from the chamber towards the outside, or vice versa.

TECHNICAL FIELD

The invention relates to a system for operating a fluid actuator, inparticular a method and system for adjusting/varying the stroke of afluid-powered actuator.

BACKGROUND

In injection molding machines, taken as an example here, molten materialis injected into a mould through one or more injection nozzles whoseopening and closing is controlled by an actuator (pin). For complexparts, produced with the multi-injection method, the positional controlof the shutter that opens and closes the nozzle is essential to obtain ahigh quality finish. In particular, it is important to be able toadjust/vary the stroke of the shutters, especially the opening strokewhich determines the residual free space between the nozzle and the tipof the shutter, and consequently determines the flow rate of thematerial and entry speed into the mold. Currently, for molding productsof value, the shutters are controlled by electric actuators, whichhowever require complex and expensive command and control devices (anelectronic control unit+related software). The use of fluid-poweredactuators is therefore less expensive but they little lend themselves toprecise stroke adjustment.

SUMMARY

The main object of the invention is to improve this state of the art.

Another object is to exploit hydraulic actuators for a positionalcontrol of the shutter, of the start and end stroke positions as well asof one or multiple intermediate stop positions or of speed variation.

An aspect of the invention relates to a method for operating an actuatoradapted for moving through a pressurized fluid the shutter of aninjection nozzle (or more nozzles) for molten material into a mold,

wherein the shutter moves from a closing position, in which there is nopassage of molten material through the nozzle, to an opening position,in which there is passage of molten material through the nozzle,

wherein the actuator comprises

a chamber, and

a piston which is movably mounted in the chamber, displaced linearlythanks to the thrust of the fluid and connected to the shutter,

with the steps of

determining a quantity of fluid before sending it to the actuator orextracting it from the actuator, and

moving the shutter by inserting or removing the predetermined amount offluid into/from the chamber.

A preferred step envisages moving the shutter by inserting or removing apredetermined amount of fluid into/from the chamber, e.g. by moving apredetermined quantity between the chamber and an auxiliary tank.

Predetermined quantity means a quantity of fluid whose volume has beendetermined or defined or calculated or set before sending it to theactuator or extracting it from the actuator. The predetermination cantake place e.g. by measuring the volume, the weight, or flow-rate, aswell as through real-time measurement/counting (e.g. through aflow-meter with the cooperation of a valve, or similar systems) of thefluid sent/extracted into/from the piston's chamber. The predeterminedamount of displaced fluid into/from the chamber ends up in aproportional linear displacement of the shutter.

In a variant, the predetermined amount of fluid is measured anddetermined before injecting it into the chamber. By means of thecalculation of the amount of fluid injected in the chamber, e.g. througha flow-meter, it is possible to determine not only the stroke of theshutter but also any intermediate position within the total stroke ofthe shutter. These intermediate positions may be multiple and theshutter can stop in those positions for a predetermined time and thencontinue the opening movement or it can reverse the direction ofmovement, for a predetermined time (downstream), with subsequent restartof the opening movement.

Preferably the predetermined amount of fluid to be moved into/from thechamber is stored elsewhere, in a volume having controlled capacity, andfrom the volume said quantity is moved into the chamber and/or viceversa. So the predetermined quantity of fluid is determined only onceand is reused in subsequent injection cycles. A throwaway system is alsopossible, wherein the predetermined quantity of fluid is sent to theshutter, expelled from it and then disposed of or mixed with otherfluid. In this case it is necessary to re-determine said predeterminedquantity of fluid at each injection.

The method applies to a unidirectional or bidirectional movement of theshutter.

In particular, the method envisages an auxiliary tank fluidly connectedto the actuator's chamber, and the predetermined amount of fluidcorresponds to a predetermined variation of fluid contained in theauxiliary tank, in particular the predetermined amount of fluidcorresponds to a predetermined variation of the volume (capacity) of achamber, comprised in the auxiliary tank, which contains the fluid.

The method applies both for a filling of the actuator's chamber and foran emptying of the actuator chamber.

In particular, the shutter is moved from a nozzle opening positiontowards a closing position by transferring said quantity from theauxiliary tank into the chamber.

In particular, the shutter is moved from a nozzle closing positiontowards an opening position by transferring said quantity from thechamber into the auxiliary tank.

The shutter opening position may correspond to an end-of-stroke positionopposite to the closing position and/or it may correspond to a positionintermediate to the end-of-stroke position opposite to the closingposition.

By subtracting a volume of fluid Vc from the fluid contained in theactuator chamber, a precise displacement of the actuator pistonproportional to Vc is obtained. In the same way, by extracting a volumeof fluid Vt from the auxiliary tank and injecting it into the actuatorchamber, a precise displacement for the actuator's piston in theopposite direction and proportional to Vt is obtained.

In particular, the quantities Vc and Vt are determined by a change involume of the chamber in the auxiliary tank. The auxiliary tank has e.g.a variable-volume chamber capable of expanding and receiving the volumeVc and capable of contracting to expel the volume Vt.

Called Vmin the minimum volume (for example in cm³) that can be reachedby the chamber of the auxiliary tank after expulsion of the fluid, andcalled Vmax the maximum volume (e.g. in cm³) that can be reached bychamber of the auxiliary tank after the injection of the fluid, it ise.g.

Vmax=Vmin+Vt and/or Vmin=Vmax−Vc.

Vmin and/or Vmax are adjustable to determine the volume change of thechamber of the auxiliary tank, hence the determination of Vc and Vt.

Vmin may be zero, but not necessarily, i.e. the chamber of the auxiliarytank does not necessarily reach zero volume after the expulsion of Vt,and may contain a residual amount of fluid. Preferably Vc=Vt, to givecyclicality to the movement of the shutter, but not necessarily.

Therefore, according to a preferred variant of the method, the stroke ofthe actuator can be adjusted/varied by modifying the Vc and Vt valuewhen needed, preferably by varying Vmax and/or Vmin.

That is, said quantity of fluid is determined by adjusting the maximumvariation of the volume of the auxiliary tank's chamber.

In the light of the above, if the geometry of the auxiliary tank varies,Vmax varies, that is, the maximum quantity that can be contained by theauxiliary tank's chamber varies.

The method does not depend on the particular hydraulic circuit used totransfer the fluid, nor on the type of fluid.

According to a preferred variant of the method, the value Vmax and/orVmin is adjusted by varying the structural configuration of theauxiliary tank.

In particular, the auxiliary tank is modified by moving a wall of aclosed cavity that delimits the volume of the auxiliary tank's chamberoccupied by the predetermined amount of fluid. The wall may be rigid,such as e.g. the surface of a plunger, or elastic.

Or the auxiliary tank is varied by spatially deforming the walls of aclosed cavity that delimits the volume of the auxiliary tank's chamberoccupied by the predetermined amount of fluid.

Or the auxiliary tank may be varied by making the walls of a closedcavity, that delimits the chamber of the volume of the auxiliary tankoccupied by the predetermined amount of fluid, expand or contract, e.g.by changing the temperature of the walls or by elastic stretching.

Or the auxiliary tank is varied by varying the amount of a fillermaterial present inside a closed cavity that delimits the chamber of theauxiliary tank's volume occupied by the predetermined amount of fluid.The volume of the filler material subtracts from the volume availablefor the fluid, and therefore Vc and Vt decrease. By removing fillermaterial the volume available to the fluid increases, and thus Vc and Vtincrease.

According to a preferred variant of the method, the auxiliary tank hasthe closed cavity or chamber delimited by a piston, and

the linear position of the auxiliary tank's piston is detected while thefluid is transferred from the auxiliary tank's chamber to a chamber ofthe actuator,an end-of-stroke position of the shutter is determined by adjusting thestroke of the piston of the auxiliary tank by exploiting theproportionality betweenchange in the position of the auxiliary tank's piston,amount of fluid transferred between the auxiliary tank and theactuator's chamber following the movement of the auxiliary tank'spiston, andvariation of the shutter's position.

Another aspect of the invention concerns a system for driving anactuator which comprises a chamber and a piston that is

-   -   movable/displaceable inside the chamber following the action of        a pressurized fluid on the piston, and    -   connected to the shutter of an injection nozzle of molten        material for injection moulding,

the system being configured to adjust/vary the shutter stroke of theactuator and comprising a device or means for forcing the transfer of apredetermined amount of fluid between the outside and inside of thechamber,

wherein the amount of fluid is determined before sending it to theactuator or extracting it from the actuator,

so as to bring/move the shutter from a closing position, in which thereis no passage of molten material, towards an opening position, in whichthere is passage of molten material, by injecting the predeterminedamount of fluid from the outside into the chamber or by injecting thepredetermined quantity of fluid from the chamber to the outside, and/or

so as to bring/move the shutter from an opening position, in which thereis passage of molten material, towards a closing position, in whichthere is no passage of molten material, by extracting the predeterminedquantity of fluid from the chamber or by sending the predeterminedquantity of fluid into the chamber, respectively.

In particular the system comprises

-   -   an auxiliary tank for fluid (preferably external to the        actuator) configured for containing a volume of fluid,    -   a circuit for fluid for putting the chamber and the auxiliary        tank into fluid communication,    -   a device or means for forcing the transfer of a predetermined        amount of fluid between the auxiliary tank and the chamber,

so as to bring/move the nozzle's shutter from a closing position, inwhere there is no passage of molten material, to an opening position, inwhich there is passage of molten material, by injecting fluid from theauxiliary tank to the chamber or by injecting fluid from the chamber tothe auxiliary tank, and/or

so as to bring/move the nozzle's shutter from an opening position, inwhich there is passage of molten material, to a closing position, inwhich there is no passage of molten material, by extracting fluid fromthe chamber and putting it inside the auxiliary tank or by extractingfluid from the auxiliary tank and putting it into the chamber,respectively.

The shutter may be part of the actuator or not.

In particular, the system comprises a device or means for determiningand/or adjusting the amount of transferred and/or transferable fluidfrom the auxiliary tank to the chamber and vice versa.

With said system it is possible to adjust e.g. the total and maximumstroke of the piston of the actuator, corresponding to the shutter'sopening end-of-stroke position, the closuring end-of-stroke position ofthe shutter being defined by a mechanical abutment of the shutteragainst the nozzle gate (conical shutting) or of the piston itselfagainst an abutment made in the body of the actuator (cylindricalshutting).

According to a preferred variant, which allows adjusting anend-of-stroke position of the piston, the auxiliary tank comprises anadjustable volume to contain the fluid to be sent into the chamber or tobe received by the chamber. In other words, referring to what has beenalready explained for the method, the auxiliary tank is configured to beable to vary its own Vmax and/or Vmin.

Preferably the auxiliary tank has a fluid capacity which is adjustable.

The fluid capacity of the auxiliary tank is adjustable in various ways.

E.g. the auxiliary tank comprises a closed cavity adapted to containsaid predetermined amount of fluid; wherein the closed cavity can reacha minimum volume Vmin and/or a maximum volume Vmax. In particular, theauxiliary tank comprises a deformable shell defining said closed cavity;wherein the shell is deformable so that the closed cavity reaches theminimum volume Vmin and/or the maximum volume Vmax.

In particular, the closed cavity may comprise

-   -   a movable wall to vary the internal volume of the cavity. The        wall may be rigid, such as e.g. the surface of a plunger, or        elastic; and/or    -   expandable or contractable walls, e.g. through means for varying        the temperature of the walls or by elastic stretching of the        walls.

Or the auxiliary tank comprises a closed cavity with a filler materialinside, and means for varying the amount of filler material in theclosed cavity, so that the variation of filler material determines aminimum residual volume Vmin and a maximum residual volume Vmax of thecavity that can be occupied by the fluid.

In a variant, the auxiliary tank comprises a piston which isdisplaceable/movable inside a chamber of the auxiliary tank followingthe action of the pressurized fluid,

the auxiliary tank's chamber being divided by the piston into

a first sub-chamber fluidically connected with the actuator's chamber,and a second sub-chamber fluidically connected to said device or meansfor forcing the transfer,

the first and second sub-chamber remaining defined on opposite sides ofthe piston,

the piston stroke being adjustable to define the maximum and/or minimumvolume of the first sub-chamber.

In particular, the system comprises mechanical, electrical and/orelectronic means for adjusting and controlling said piston's stroke(e.g. linear encoders, rotary encoders, laser or infrared readingsystems, mechanical screw stops, screw/nut screw systems, optic fiber,switches, microswitches, etc.).

Preferably the system comprises an electronic control unit forcontrolling the system's components, especially the piston and thedevice or means for forcing the transfer of the predetermined amount offluid, and the means cited in one or each of the claims.

The adjustment or control means are preferably connected to the controlunit to give the real-time position of the piston. Based on suchposition the control unit is able to determine/vary the movement of thepiston and consequently of the shutter (it is possible to vary the stopposition, the movement speed of the shutter by increasing or decreasingit, the elapsed time when stopped, etc.).

According to a preferred variant of the method and of the system, theauxiliary tank has the closed cavity or chamber delimited by a piston,and in particular said movable wall in the closed cavity mayadvantageously be a surface of the aforesaid piston.

Said wall or piston in the auxiliary tank may e.g. be movable betweentwo positions, and the volume swept by the wall determines the volume ofsaid predetermined amount of fluid. In particular, the wall or piston ofthe auxiliary tank is mounted in such a way that at one of said twopositions it abuts against a movable abutment element. The position ofthe movable abutment element is adjustable and one can then adjust theposition of one of said two positions, and hence the volume of saidpredetermined amount of fluid. In a variant, the movable abutmentelement is an element screwable in the auxiliary tank's body.

Preferably the auxiliary tank comprises a locking element for blockingthe position of the movable abutment element.

E.g. a linear position sensor can communicate with the control unitwhich processes the data outputted by the sensor, showing the positionof the shutter on a display, and allowing a human operator to verify thecorrect functioning of the system inside the mold.

In a variant, the circuit comprises

-   -   a first fluid transport line between the chamber and the        auxiliary tank,    -   a second fluid transport line between the chamber and the means        or device for forcing the transfer of fluid,    -   a third fluid transport line between the auxiliary tank and the        means or device for forcing the transfer of fluid, wherein

the first and second lines open into the actuator's chamber forinjecting into and/or extracting fluid from opposite sides of thepiston, the third line carrying fluid different and isolated from thatwhich flows in the first line.

In particular, the circuit comprises

-   -   a first fluid transport line between the chamber and the first        sub-chamber,    -   a second fluid transport line between the chamber and the means        or device for forcing the transfer of fluid,    -   a third fluid transport line between the second sub-chamber and        the means or device for forcing the transfer of fluid, wherein

the first and second lines open into the actuator's chamber to injectinto and/or extract fluid from opposite sides of the piston,

the third line carrying fluid different and isolated from that whichflows in the first line and opening into the second sub-chamber of theauxiliary tank to inject or extract fluid on one side of said twoopposite sides of the auxiliary tank's piston.

The direction of displacement/movement of the fluid from the auxiliarytank to the actuator, like the related connections, may be invertedwithout changing the effects and advantages of the invention (e.g. likeinverting the inlets of the line 19 a with line 19 b, see the followingfigures).

Preferably, there are means for putting the two sub-chambers intofluidic communication, for initial fillings or fluid vents.

In a variant, the volume variation of said closed cavity or chamber ofthe auxiliary tank (e.g. the fluid transfer to the second sub-chamber)takes place by pushing the fluid through an air or gas circuit.

In a variant, the system comprises means for varying the fluid presentin one of the two fluid transport lines (therefore in chambers 18 a and18 b, see below),by using for example air or gas (compressible fluids).The variation of the fluid involves the addition or modification of someof the circuit components, but the principle of operation does notchange.

In a variant, the system comprises a mechanical (e.g. spring) system forobtaining the movement of the shutter in the actuator in one direction.

Preferably the system comprises leaks or overflows for initially fillingthe first line with fluid or compensating for any fluid leakages.

Another aspect of the invention relates to a software for performing oneor each of the method steps by controlling the fluid flow.

The invention is applicable e.g. to systems with shutter having conicalor frusto-conical end cooperating with a gate having a cavity ofcomplementary shape, or to systems with shutter having a cylindrical endcooperating with a gate having a cavity of complementary shape.

The invention is also applicable e.g. to systems where the auxiliarytank supplies multiple actuators; and/or in which an actuator controlsseveral shutters simultaneously (the actuator's piston is integral withmultiple shutters).

A variant of the invention offers the advantage of being able to monitorthe shutter's position and/or speed and/or acceleration indirectly,without acting directly on the actuator fixed on the hot runner insidethe mold. Indeed, it should be noted that the instantaneous position ofthe shutter can be derived by measuring in real time, for example with aflow-meter,

the quantity of fluid that passes into/from the auxiliary tank,

the fraction of said predetermined quantity of fluid that has arrived inthe shutter chamber and/or that has come out,

the position of said wall or piston,

The position of said wall or piston may be advantageously monitored e.g.by a position sensor, e.g. a linear sensor (of various types:potentiometric, optical fiber, magnetic, laser, etc.). The sensor thenallows indirectly monitoring the instantaneous position of the shutter'spiston.

This aspect also highlights a diagnostic application of the above methodor system, i.e. to detect any malfunctions of the hydraulic system oroil leaks or overflows cycle after cycle. The above method or system canbe applied to a pre-existing plant to verify the correct driving of ashutter during the injection phase.

To obtain maximum precision, said fluid is preferably a liquid, e.g.oil. The liquid is incompressible and guarantees proportionality betweenthe volume of displaced fluid and the linear displacement of theactuator's piston.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages of the invention will be even clearer from the followingdescription of a preferred system, in which reference is made to theattached drawing in which

FIG. 1 shows a diagram of the system;

FIGS. 2 and 3 show a scheme of variants of the system.

In the figures, like elements are indicated by same numbers.

DETAILED DESCRIPTION

The MC system of FIG. 1 is used to control an actuator 10.

The actuator 10 is fixed on a support, such as for example a hot runner90 (manifold) or a plate or a mold, and is intended for thedisplacement/driving of a shutter 12 for an injection nozzle 13.

The shutter 12 is connected to a piston 14 which is linearly movableinside a chamber 18 defined by a casing 16.

In the example of FIG. 1 the chamber 18 is divided by the piston 14 intoa first chamber 18 a, communicating with an inlet 19 a, and a secondchamber 18 b, communicating with an inlet 19 b. The chamber 18 a is e.g.isolated from the chamber 18 b.

Through the fluid inlets 19 a, 19 b it is possible to inject or extractfluid, e.g. oil, respectively into the chamber 18 a or 18 b on oppositesides of the piston 14, so as to be able to move it linearly in oppositedirections. Therefore, by injecting and extracting alternatively fluidin the chambers 18 a and 18 b the piston 14 can be moved, andconsequently the shutter 12. The movement of the latter determines theopening or the closing of the nozzle 13 with the consequent passage orobstruction of the molten material. The closing end-of-stroke positionof the shutter 12 is established by the mechanical abutment of theshutter 12 against the so-called “gate” of the nozzle 13 so that thereis no passage of molten material, or a suitable end-of-stroke abutmentis provided between the piston and the body of the actuator 10.

The stroke of the shutter 12 from the closing position to the maximumopening position or towards the opening end-of-stroke position is, as itwill be seen below, adjustable.

A tank 60, external to the actuator 10, comprises an external casing 62with inside a piston 64 linearly movable inside a chamber 66 defined bythe casing 62. The piston 64, like the piston 14, divides the chamber 66into two sub-chambers 66 a, 66 b.

A first line 20 allows transferring fluid, through the inlet 19 a, fromthe chamber 18 a to the chamber 66 a and vice versa, while a second line22 allows transferring fluid, through the inlet 19 b, from the chamber18 b to the chamber 66 b and vice versa.

The second line 22 is connected and cooperates with a third line 52connected to a pump (not shown) that allows the fluid to circulate on afourth line 50 to make it return through a fifth line 24 to the chamber66 b of the tank 60.

Through the first line 20 the fluid is injected into the chamber 18 aagainst one side of the piston 14, while through the line 22 the fluidis injected into the chamber 18 b on the opposite side of the piston 14.Similarly, fluid is injected through the line 20 into the chamber 66 aagainst one side of the piston 64, while through the line 24 the fluidis injected into the chamber 66 b on the opposite side of the piston 64.By injecting fluid from the line 24 into the chamber 66 b the piston 64is pushed against the chamber 66 a.

The fluid in the line 24 is isolated from that in the line 20, toconserve a constant quantity of fluid in the chamber 66 a, but the fluidchanges in the lines 20, 22, 52, 50 and 24 are substantially equal, sothat they actually form a closed fluid circuit.

Preferably, leaks or overflows are provided to initially fill with fluidthe line 24, the chamber 66 a and the chamber 18 a, or to compensate forany small leakages of fluid during operation.

The tank 60 is equipped with manual or servo-assisted means 68 forregulating or varying the maximum amplitude of the stroke of the piston64 towards the chamber 66 b, in order to adjust the maximum volume ofthe chamber 66 a (or-which is the same-the minimum volume of the chamber66 b). The means 68 may be made e.g. with a mechanical abutment 98between the piston 64 and the casing 62, where the position of theabutment point or of the mechanics abutment 98 is adjustable along thetranslation axis of the piston 64 (see arrow F). Preferably there is alocking means 96 for blocking the position of the mechanical stop 98after it has been selected.

The tank 60 is preferably provided with means (not shown) for detectingthe linear position of the piston 64, for a remote and automaticposition control and/or for a precise position regulation. E.g. theposition of the piston 64 is detected by a linear encoder, a Hallsensor, a mechanical gear/nut screw system, etc.

In the lines 24, 22 there are also inserted optional flow regulators 30,32, which allow, in addition to a homogeneous movement, the setting ofdifferent movement speeds for the piston 14.

A fluid diverter 40, having a movable distributor 42, serves to reverse,at each complete cycle, the flow direction of the fluid in the circuit,which determines the direction of movement of the shutter 12, uponopening or closing.

Operation

By injecting fluid into the chamber 18 a, through the action of thepump, from the line 20, the shutter 12 can be moved from the openingposition to the closing position, or towards the closing end-of-strokeposition (downwards in FIG. 1 ).

In this phase the total quantity of fluid Q1 injected into the chamber18 a from the line 20 coincides with a change in the amount of fluidstored in the chamber 66 a. Such variation in the quantity of fluid canbe determined e.g. by adjusting the stroke-limit means 68 of the piston64 when it moves towards (and against) the chamber 66 a, or it isautomatically determined by the maximum volume that the chamber 18 areaches in correspondence of the closing end-of-stroke position. Or theaforementioned change in quantity of fluid is determined/processed bythe control unit on the basis of the detection, through suitable meansnot shown (e.g. a linear encoder), of the actual position of the piston64. In this way the volume variation of fluid in motion can be changedremotely (via a tablet or the like) without the need to act next to themachine/press. The linear encoder, or similar means, is also suitablefor detection and control of intermediate stop positions for theshutter.

It is not necessary for the piston 64 to end up nulling the volume ofthe chamber 66 a.

By injecting fluid into the chamber 18 b, through the action of thepump, from the line 22, the shutter 12 can be moved from the closingposition to the opening position, or towards the opening end-of-strokeposition (upwards in FIG. 1 ). The fluid coming from the line 22 pushesthe piston 14 against the chamber 18 a. The piston 14 in turn pushesfluid out of the chamber 18 a and into the chamber 66 a, to move thepiston 64 in the opposite direction. In this phase, the quantity offluid Q2 which can be moved from the chamber 18 a to the chamber 66 a isdetermined by the stroke-limit means 68 of the piston 64 when it movestowards the chamber 66 b.

It is Q1=Q2, wherein the amplitude of the backward stroke of the piston64 establishes what is the volume of displaced fluid Q2.

The backward stroke of the piston 64 is adjusted by the means 68 todefine the volume/quantity of fluid to be moved, inserted and/orextracted from/to the chamber 18 a, in order to obtain the desired, e.g.opening, stroke for the shutter 12; e.g. a stroke of 10 to 40 mm, e.g.25 mm.

By acting on the means 68, Q1 and Q2 can be varied.

The geometry of the system ensures that a change of fluid Q1, Q2 in thechamber 18 translates into a corresponding displacement of the piston14. The value Q1, Q2 then determines a constraint on the motion of thepiston 14 inside the chamber 18, thereby establishing the strokeamplitude and/or the opening end-of-stroke position thereof.

Preferably, the maximum volume of the fluid contained in the chamber 66a is always greater than the maximum volume of the chamber 18 a, so thatthere is a reserve of fluid necessary to compensate for any leaksbetween the various branches of the fluid circuit.

Note that to reverse the movement of the shutter, the line 20 can supplythe inlet 19 b and the line 22 can supply the inlet 19 a.

Variants

The described system and method can control the shutter for a singlenozzle or, according to the same logic, could control simultaneously twoor more nozzles with the same actuator, see FIG. 3 .

Or (FIG. 2 ) more shutters 12 a, 12 b, 12 c (in the example, three) withrelative different actuators 10 a, 10 b, 10 c are connected to the tank60 through by-passing lines 20, 22 to be fed with fluid therefrom. Thevolume of fluid moving in the circuit will be proportionally modified,with suitable means not shown, by dividing among the various actuatorsthe volume of displaced fluid, essential in sequential injectionsystems.

Of course, the invention can also be applied, without substantialmodifications, in systems wherein the actuator and/or the external tankrequires the use of a multiple-stage and/or multiple-chamber piston (seeWO2004027302). In the variant shown in FIG. 1 , the external tank 60exhibits a piston 64 movable inside a chamber 66 thanks to the thrust ofthe fluid. In a variant, the piston 64 can be operated from the outsideof the tank 60 in a controlled manner, so that the piston 64 follows adynamic reference (e.g. a speed and/or position reference) to controlthe dynamics of the shutter 12 accordingly.

E.g. the piston 64 can be operated by an electric gearmotor controlledby an electronic control unit, through which various dynamic profilesare applicable to the piston 64.

To balance the pressure between the chambers 66 a, 66 b and/or expel anyair pocket that can form during the initial filling of the fluid, the MCsystem preferably comprises means for selectively placing the chambers66 a, 66 b into communication with each other.

In particular, the MC system comprises for this purpose a valve 200arranged on the head of the piston 64. The valve 200, when open, allowsthe passage of fluid between the chambers 66 a and 66 b, while, whenclosed, it does not allow this passage. During the whole reciprocatingmovement of the piston 64 the valve 200 remains closed, while it ismounted so as to get opened when the piston reaches the end-of-strokeposition corresponding to the maximum shrinkage of the chamber 66 b.that is, only when the upper dead point of the shutter 12 is reached. Atsuch point the valve 200 is pushed and opens upon contact with theinternal surface of the the casing 62, thus allowing the outflow offluid from the chamber 66 b to the chamber 66 a.

Preferably the MC system comprises a manual opening system for the valve200, advantageous in the installation phase, which in particularcomprises means for pushing the head of the piston 64 against theinternal surface of the casing 62. E.g. the movable abutment element 98can be made integral with the piston 64 and exploited for dragging thepiston 64 towards an end-of-stroke position and opening the valve 200.

An appropriate programming of the injection cycle allows the controlunit to command/drive the described components in order to achieve anyof the opening and/or closing profiles described and shown in thefollowing documents:

PCT/IB2019/053936, IT102017000037002, IT102016000080198,IT102016000055364, IT102015000008368, ITT02014A001030, ITT02014A001021,ITT02014A000701, WO2012/074879A1, WO2012/087491A1, WO2018/020177A1.

1. System for driving an actuator which comprises: a chamber, a shutterof an injection nozzle for molten material for injection molding, and apiston that is movable/translatable inside the chamber following theaction on the piston of a pressurized fluid, and connected to theshutter, the system being configured to adjust/vary the stroke of theshutter and comprising a device or means for forcing the transfer of apredetermined amount of fluid between the outside and inside of thechamber, wherein the amount of fluid is determined before sending it tothe actuator or extracting it from the actuator, so as to bring/move theshutter from a closing position, in which there is no passage of moltenmaterial, towards an opening position, in which there is passage ofmolten material, by injecting the predetermined amount of fluid from theoutside into the chamber or by injecting the predetermined amount offluid from the chamber towards the outside, and/or so as to bring/movethe shutter from an opening position, in which there is passage ofmolten material, towards a closing position, in which there is nopassage of molten material, by extracting the predetermined amount offluid from the chamber or by injecting the predetermined amount of fluidinto the chamber, respectively.
 2. System according to claim 1,comprising an auxiliary tank of fluid configured to contain a volume offluid, a circuit for fluid for putting the chamber and the auxiliarytank into fluid communication, a device or means for forcing thetransfer of a predetermined amount of fluid between the auxiliary tankand the chamber, so as to bring/move the shutter from a closingposition, in which there is no passage of molten material, to an openingposition, in which there is passage of molten material, by injectingfluid from the auxiliary tank towards the chamber or injecting fluidtowards the auxiliary tank, and/or so as to bring/move the shutter froman opening position, in which there is passage of molten material, to aclosing position, in which there is no passage of molten material, byextracting fluid from the chamber and putting it inside the auxiliarytank or by extracting fluid from the auxiliary tank, and putting itinside the chamber, respectively.
 3. System according to claim 2,comprising a device or means for determining and/or adjusting thequantity of fluid that can be transferred from the auxiliary tank to thechamber and vice versa.
 4. System according to claim 2, wherein theauxiliary tank comprises an adjustable volume for containing the fluidto be sent to the chamber or to be received from the chamber.
 5. Systemaccording to claim 4, wherein the auxiliary tank comprises an adjustablevolume for containing the fluid to be sent to the chamber or to bereceived from the chamber.
 6. System according to claim 2, wherein theauxiliary tank comprises a closed cavity adapted to contain fluid; theclosed cavity being adapted to reach a minimum volume (Vmin) and amaximum volume (Vmax) through a movable wall for varying the internalvolume of the cavity.
 7. System according to claim 6, wherein said wallis movable for stopping against an abutment element, the abutmentelement being configured to have an adjustable position in order toadjust the stroke of the wall.
 8. System according to claim 7,comprising an element for blocking the position of the abutment element.9. System according to claim 6, wherein the auxiliary tank comprises adeformable shell defining said closed cavity; wherein the shell isdeformable so that the closed cavity reaches the minimum volume Vminand/or the maximum volume Vmax.
 10. System according to claim 6, whereinthe auxiliary tank comprises two chambers for fluid isolated andseparated from the movable wall, each chamber of the auxiliary tankbeing fluidically connected to a respective chamber for fluid of theactuator, so that the displacement of the movable wall sends apredetermined amount of fluid from the first chamber of the auxiliarytank to the first chamber of the actuator, and a movement in oppositedirection of the movable wall sends a predetermined quantity of fluidfrom the second chamber of the auxiliary tank to the second chamber ofthe actuator.
 11. System according to claim 6, comprising a valvearranged on the movable wall, the valve being configured so that, whenopen, it allows the passage of fluid between the chambers, while whenclosed it does not allow such passage.
 12. System according to claim 2,wherein the auxiliary tank comprises a piston which isdisplaceable/movable inside a chamber of the auxiliary tank followingthe action of the pressurized fluid, the auxiliary tank's chamber beingdivided by the piston into a first sub-chamber fluidically connectedwith the actuator's chamber, and a second sub-chamber fluidicallyconnected to said device or means for forcing the transfer, the firstand second sub-chamber remaining defined on opposite sides of thepiston, the piston stroke being adjustable to define the maximum and/orminimum volume of the first sub-chamber.
 13. System according to claim12, comprising an electronic control unit for controlling the piston inthe auxiliary tank and the device or means for forcing the transfer ofthe predetermined amount of fluid.
 14. System according to claim 12,comprising a position sensor for monitoring the instantaneous positionof said wall or piston in the auxiliary tank.
 15. System according toclaim 2, comprising means for monitoring the shutter's position and/orspeed and/or acceleration thereof by measuring in real time, thequantity of fluid that passes into/from the auxiliary tank, or thefraction of said predetermined quantity of fluid that has arrived in theshutter chamber and/or that has come out, or the position of said wallor piston in the auxiliary tank.
 16. System according to claim 2,wherein said circuit comprises a first fluid transport line between thechamber and the auxiliary tank, a second fluid transport line betweenthe chamber and the means or device for forcing the transfer of fluid, athird fluid transport line between the auxiliary tank and the means ordevice for forcing the transfer of fluid, wherein the first and secondlines open into the actuator's chamber for injecting into and/orextracting fluid from opposite sides of the piston, the third linecarrying fluid different and isolated from that which flows in the firstline.
 17. System according to claim 12, wherein said circuit comprises afirst fluid transport line between the chamber and the firstsub-chamber, a second fluid transport line between the chamber and themeans or device for forcing the transfer of fluid, a third fluidtransport line between the second sub-chamber and the means or devicefor forcing the transfer of fluid, wherein the first and second linesopen into the actuator's chamber to inject into and/or extract fluidfrom opposite sides of the piston, the third line carrying fluiddifferent and isolated from that which flows in the first line andopening into the second sub-chamber of the auxiliary tank to inject orextract fluid on one side of said two opposite sides of the auxiliarytank's piston.